Automatic compression release mechanisms are employed in internal combustion engines to provide for improved engine performance at a variety of engine speeds. Such mechanisms typically include a component which is actuated based upon engine speed, that varies an exterior surface characteristic of a cam lobe along which mating valve train components actuate exhaust and/or intake valves of the engine. When the engine is cranking, a protrusion is created on the cam lobe such that the exhaust valve opens slightly during the compression stroke of the engine. The reduced compression caused by this “low speed orientation” reduces the effort to start the engine. However, when engine speeds are higher, such as during normal operation or idling, the protrusion is eliminated such that the exhaust valve remains closed during the compression stroke of the engine. This “normal speed orientation” maximizes engine power.
Automatic compression release mechanisms of this type often employ a weight assembly that is rotatably affixed to a portion of the camshaft such as a cam gear. As the camshaft rotates, centrifugal forces acting on the weight cause the weight to move radially outwards, away from the camshaft axis. However, the weight is typically biased by a spring towards the camshaft so that when the engine is at low speeds, the weight is pulled inward toward the camshaft. Because the movement of the weight is dependent upon the rotational speed of the camshaft, the movement of the weight can be used to govern components associated with the cam lobe to produce the desired speed-dependent variation in cam lobe shape. Commonly these components include a contoured shaft having a recessed side and an unrecessed side, which is coupled to the weight. The contoured shaft is disposed in a notch formed in the surface of the cam lobe, and when the weight is disposed radially inwards at low engine speed, the unrecessed side of the contoured shaft extends outward beyond the exterior surface of the cam lobe producing a protrusion. When the weight is rotated outwards at higher engine speeds, the recessed side of the contoured shaft faces outward and the protrusion on the cam lobe is largely or entirely eliminated.
In many engines, it is desirable to employ an automatic compression release mechanism having as few components as possible, in order to simplify and consequently reduce the costs of the mechanism. This can be achieved to some extent by integrally forming as a single piece assembly the weight and the contoured shaft such that rotation of the weight directly causes rotation of the contoured shaft. For similar cost-related reasons, it often is desirable for engines to employ simply-formed and inexpensive components throughout the cam shaft assembly. For example, the cam gear can be molded out of plastic or die cast as a single piece. Also, the cam lobe can be integrally formed as part of the cam gear, or at least fixedly attached to the cam gear.
When shutting down any engine, its rotation is slowed both by friction and by the work of the piston against gasses in the cylinder during the compression stroke. During this shut down the contoured shaft rotates to the low speed orientation in which the protrusion is exposed on the cam surface. If at the final moments of rotation there is insufficient angular momentum to accomplish the compression event, however, the compressed gas will work against the piston to cause a small amount of reversed rotation. This small reversed rotation of the engine can cause the cam follower to bear against the recessed, or flat side of the contoured shaft and rotate it against the bias spring force to its normal speed orientation. The automatic compression release mechanism thus becomes disabled for the subsequent starting event, thus making it difficult to restart the engine due to the high compressive forces.